As you have guessed, all small sibs use shortshaft outboards. A couple of the larger and newer Zodiac Futura's use long shafts, but they wouldnt be much fun with a 6hp motor.
Long shafts are not ideal on shortshaft transoms, the 2 biggest issues are stability and drag when on the plane and also occasionally too much back pressure on the exhaust resulting in poor starting. About 20 years ago I did use the long shaft auxillary from my hardboat on a Zodiac C240 slat floor tender. The boat wouldnt plane, but with a 2" shim on the transom it ran fine at displacement speeds with no stability issues.
FWIW I would give it a go, worst case you have to sell the sib for a few quid loss.